fix solo when using layers

[adamgayoso]

Fixes #1095

[codecov]

Codecov Report

Merging #1098 (c70d327) into master (527fcea) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1098   +/-   ##
=======================================
  Coverage   90.64%   90.65%           
=======================================
  Files          91       91           
  Lines        6884     6889    +5     
=======================================
+ Hits         6240     6245    +5     
  Misses        644      644           

Impacted Files	Coverage Δ
scvi/external/solo/_model.py	94.26% <100.00%> (+0.24%)	⬆️

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